In Harper Lee's To Kill a Mockingbird, Tom Robinson, a black man, is accused of raping and attacking Mayella Ewell, a poor white woman and the daughter of the town drunk, Bob Ewell. The violent nature of this case causes quite a sensation in Maycomb, and the entire town shows up to the courthouse at the beginning of the trial to watch its proceedings.
Atticus Finch, who has been appointed by Judge Taylor as Tom's attorney, calls Bob to the stand as the second witness in the trial. Atticus asks Bob if he is able to write, and Bob confidently tells Atticus that he is, as that is how he signs his "relief checks." Bob writes down his name in front of the court, which demonstrates one huge discovery: he is left-handed.
Due to the fact that Mayella's injuries were all on the right side of her face, one could deduce that a left-handed man had to have attacked her. It is soon thereafter revealed that Tom has a mangled left arm due to a childhood accident; he could not possibly have been the one to attack Mayella. Simply put, the girl is lying.
The second witness in the court scene was Bob Ewell. It was his testemony that proved he was left handed and proved Tom Robinson innocent because Tom's left hand was mangled and crippled.
Atticus Finch purposely called Bob Ewell to the stand because he wanted to show before the entire court that he signed his name with his left hand.